Stem cell therapies play a major and increasing role in cancer therapies. The ability to expand stem cell numbers would have broad application in hematology and oncology. In addition, the ability to enforce stem cell differentiation could improve the therapy of many cancers, notably acute leukemias. Biochemical and genetic experiments in many systems indicate that homeobox proteins regulate the balance between stem cells and lineage-specific cells, in many tissues. We have recently found that the trimeric transcription factor NF-Y, which had been previously shown to regulate c-jun, p27 and CD34 transcription, is the key regulated transcription factor controlling the expression of HOXB4, as well as the paralogs HOXC4 and HOXD4. Enforced overexpression of NF-Ya, the inducible element of the trimer, in stem cells by retroviral gene transfer increases the expression of HOXB4, HOXC4, HOXD4, as well as hTERT, LEF-1 and several stem cell markers. NF-Ya overexpression in stem cells also increases stem cell numbers as assayed by competitive repopulation following stem cell transplantation, while inhibition of NF-Y activity decreases stem cell numbers and promotes terminal differentiation. Based upon these data, we hypothesize that NF-Y functions as a master switch, controlling the expression of several genes critical for stem cell cycling and proliferation, including homeobox and other genes. To explore and test this hypothesis, we propose to: 1) Measure the consequences of NF-Ya over/ and underexpression in hematopoietic stem cells, both in normal mice and mouse strains deficient in each of several candidate downstream NF-Y target genes;2) test the ability of soluble TAT-NF-Ya protein to biochemically and reversibly activate NF-Y target genes and increase HSCs, as measured by transplantation in vivo;and 3) Test the ability of DN-NF-Ya, and TAT-DN- NF-Ya protein to differentiate primary AML blasts in vitro, and on NOD/SCID mice with human AML in vivo. These experiments will describe in detail the role of NF-Y in the biology of hematopoietic stem cells, and will develop two experimental therapies based directly on the biology of NF-Y, for both HSC expansion and HSC differentiation.